In plumbing installations, copper tubing is widely employed. In risers, used for connecting tubing to fixtures or tanks, the end of the copper tubing is shaped to form a bulb sealing surface and such bulb includes a shoulder permitting the tubing and the bulb sealing surface to be drawn into biting or sealing engagement with the fixture. The cost of such copper tubing and the cost of forming the same to permit the connection to such fixtures or tanks is substantial.
In the past, polymers such as polybutylene were used in plumbing. Tubing or pipe made of polybutylene is normally joined by heat-fusion techniques, by mechanical compression, overmolding and by cold flaring. In order to provide such polybutylene tubing with a bulb sealing surface or an end cap for such purposes, a variety of techniques were employed. Two commonly employed techniques were: (1) spin-welding a separately molded bulb onto the outer diameter (O.D.) of the end of a tube; or (2) insert molding a bulb onto the O.D. of the end of a tube. All such processes have cost and performance drawbacks. In the spin welding technique, excessive clamping pressures may cause the loaded part to become dislodged or separated from the O.D. of the tubing and the interface of the parts provides a possibility of leakage. In the case of a neoprene or like washer employed on the O.D. of the tubing, depending on the configuration of the tube/washer interface, the same leakage susceptibility is potentially present. Moreover, a flange formed to receive the washer may itself create a point of weakness if excessive clamping pressures are employed. Further neoprene washers are known to deteriorate with age and temperature exposure. Lastly, insert molding forces hot material over a cold tube surface, creating a bond that is hard to predict and control.
One solution to this problem of providing polybutylene tubing with an attached bulb sealing surface of unitary construction is detailed in U.S. Pat. Nos. 4,316,870, 4,446,084 and 4,525,136, which are hereinby incorporated fully by reference. The thrust of these references however, is to teach the ability to maintain a constant diameter opening within the tubing. This is of necessity, due to the configuration of the mold cavity, and insertion of the mandrel inside the tubing during some of the processing steps.
After solving the sealing surface issues, the ability to bell an opposed end of the tubing, without any accompanying wall thickness compromise, which would make the product unsuitable for all plumbing applications, was addressed. Prior art solutions to the formation of a bell on one end of polybutylene tubing is by heating a portion of the end of the tubing, followed by insertion of a mandrel into the heated open end, the O.D. of the mandrel being matched to the targeted inner diameter (I.D.) of the tubing. While this approach will bell the tubing, it is incapable of reproducibly making tubing product with a constant wall thickness of 0.062"+0.010" throughout the belled end, particularly in the neck region of the bell. This is due to the fact that the bell is made by expanding the I.D. and thus thinning the walls. A solution to this problem is found in U.S. Pat. No. 5,527,503, the teachings of which are hereinby fully incorporated by reference.
The trend today however, is to shift from thermoplastic materials, e.g., polypropylene, polybutylene, etc., to combined thermoplastic/thermoset materials, e.g., crosslinked polyethylene wherein at least a portion of the polymer is crosslinked, for example approximately 65% thermoset/35% thermoplastic. However, this shift in materials is not simple in that there are several processing changes which must be incorporated in order to fabricate acceptable parts. Since thermosets in general, cannot be extruded like thermoplastics, differing processing conditions must be employed in different sequences in order to achieve similar functionality for the thermoset/thermoplastic product. While thermoplastic material can chemically bond to itself, as the percentage of crosslinking increases, there is less thermoplastic remaining to form this chemical bond. Therefore, one of the keys to this invention is the recognition of the need to form overmolded ends at the earliest time when they are the least crosslinked. When crosslinking using radiation, this is before any crosslinking occurs. With silane crosslinking, this is typically after extrusion, but before crosslinking is complete. In a preferred embodiment, the tube and the overmolded plastic will both be essentially about 35% crosslinked, and subsequently permitted to complete the crosslinking process after overmolding.
However, in many plumbing applications, a need exists for a connector wherein the characteristics of the nose cone are different from those of the tube, yet still retain the leak-proof aspects of a one-piece connector. It is desirable to produce a riser tube which is rigid enough to withstand pressures in plumbing, but which has a nose cone which is less flexible than the tube which permits sealing engagement in a leak-proof manner with a fitting. Alternatively, it is also desirable to produce a riser tube which is flexible enough to permit installation in a tight geometric configuration, yet still have a nose cone which is sufficiently rigid to permit sealing engagement in leak-proof manner with a fitting. Typically, tubing rigidity and nose cone flexibility have always been properties which were antagonistic to each other. By using the technology described in this application, it is possible to custom tailor the needs of the tubing and nose cone, which depending on the end application, may require that the rigidity of the tubing and nose cone may be essentially the same.